In order to prepare thick-film resistors, a mixture of an appropriate resistor material, a ceramic or glass binder and an appropriate vehicle is screen printed on a substrate. The resistor pattern on the substrate is then fired at a relatively high temperature, typically between 650.degree. and 900.degree. C. As the temperature rises to the firing temperature, the vehicle is volatilized, leaving the resistance material and binder behind. At the firing temperature, sintering takes place to a greater or lesser extent, with the binder providing adhesion between the resistor material and the substrate.
All known thick-film resistor systems which are compatible with polymer conductors and the like depend on contact between particles of the resistor material, which is held by the polymeric binder. The resistance value of the system is dependent on the materials incorporated into the binder. Thus, if a high resistivity material is incorporated into the polymer, the resulting resistor will have a high resistance. Alternatively, a high resistivity material can be obtained by incorporating a relatively low resistivity material in the polymer together with a portion of an inert filler material, such as silica. The filler acts to decrease the percentage of particle-to-particle contact between the particles of low resistivity, resulting in an overall high resistance material. A second alternative is to use a relatively high resistivity material and a high conductivity filler, such as silver or platinum; such a formulation will result in a resistor of low resistivity.
In the past, systems employing fillers have been used to obtain families of resistor "inks", i.e. a series of thick film resistor materials having different resistance values as a result of varying the type of filler and amount of filler. A basic problem with this approach is that the resistivity of the filler material is vastly different from the resistivity of the basic resistance material. For example, for all practical purposes, silica has an essentially infinite resistance while silver or platinum have essentially no resistance. The resistance of the composite resistor ink is highly dependent on inter-particle contact. As a result, the stability of the resistor system is adversely affected.
Resistor systems which are compatible with polymer conductors are based on polymeric binders. However, a polymeric binder has the deficiency that the compressive forces created by and within the binder are dependent on temperature and humidity conditions, which can vary considerably from time to time and from place to place. It will therefore be appreciated that a resistor system which depends on varying the amount of inter-particle contact between highly conducting or non-conducting species and a bulk resistive species is inherently unstable in the presence of variations in polymer characteristics with temperature and humidity.
Another problem with thick-film polymer resistor materials is that of cost. At the present time, virtually all of such materials are based on mixtures of ruthenium oxide and various amounts of silver, palladium and platinum. These materials are all rare and are characterized as precious and are, therefore, very expensive. For example, a typical ruthenium oxide resistor paste presently costs somewhere in the neighborhood of about $2.00 per gram.
Carbon is a material of known resistivity and carbon composition resistors have been used in electronic circuits for essentially the last century. Resistor inks based on carbon are commercially available. Using carbon in some polymer systems, however, does give rise to problems because the carbon tends to react with the free radical cure mechanism of the polymeric system and prevent proper curing of the binder. Powdered graphite can be used as a substitute in these systems. The preparation of a family of resistor inks based on carbon or graphite is difficult, however, because of the aforementioned problems associated with the filler materials. In addition, carbon resistors of all types are characterized by having poor temperature coefficients. Moreover, degradation of the stability of the resistor results from the softness of carbon and the ability to change the amount of contact surface between particles as a result of relatively small amounts of expansion and contraction of the polymer resistor system.
Accordingly, it is an object of this invention to provide a family of resistor inks having various resistance, in which optimal loading of a polymeric binder with resistive material, without the use of high conductivity or inert fillers, can be realized, and having relatively high stability when subject to temperature variations, humidity variations and atmospheric oxidizing conditions. The invention is based on low cost materials, and is dependent on the bulk resistivity properties of the resistor material and less dependent on surface properties which can vary with changes in ambient conditions. The resistive material is sufficiently hard to resist shape changes which tend to result in loss of particle contact when there are changes in particle-to-particle pressure brought about by natural variation of ambient conditions. These and other objects of the invention will become apparent to those skilled in the art from the following detailed discussion.